1. Field of the Invention
The invention relates to the field of protection. More particularly, the present invention relates to a method and apparatus for protecting busbars.
2. Discussion of Background
Busbar protection systems have been described in the book "Schutztechnik in Elektroenergiesystemen" ("Protection practise in electric power systems") by H. Ungrad et al., Springer Verlag 1991, on pages 182 ff.
In a typical busbar protection system, the currents in the outgoers of the busbar are detected with the aid of current transformers. If, a short-circuit occurs inside or outside the protection zone of the busbar, the resulting high currents occur can cause a magnetic core saturation of the current transformers. This can lead to a malfunction of the busbar protection system. In prior analog busbar protection systems, current transformer saturation was detected and the protection system blocked using analog technology.
However, due to the current transformer saturation, the system described above does not distinguish between an external and an internal fault.
A different approach is disclosed in the INX2 busbar protection system (see Brown Boveri Mitteilungen (Brown Boveri News), Volume 53, No. 4/5, pages 326 to 339) from the BBC company. Here, a voltage proportional to the current is converted into a rectangular signal by means of so-called shapers. One shaper is provided for each half wave of current for both directions. In order to reduce the influence of the current transformer saturation, an integrator is connected downstream of the shapers which holds the digital rectangular signals in their active position during a certain time. As a result of the integrator, for each saturation of a main transformer, even if it is so strong that the current breaks down before the actual zero crossing, the direction of the started half wave is stored until either the measurement is terminated or the next half wave begins. By comparison the directions or phases of the outgoer currents, faults can be detected and to identified as an internal or an external fault.
This system is a very complicated analog system which, in particular, can be used only for the protection algorithm of the phase comparison. Such a system is also described on pages 194 ff of the previously cited textbook.